unix: remove darwin support from mksysnum.go

Since CL 250437 this package no longer supports non-libc, direct
syscalls on darwin. Thus support for generating zsysnum_*.go files on
darwin can be removed from the generating mksysnum.go program.

Change-Id: I8916ddcd028a60d876ad43b8ce18a1c0925ce272
Reviewed-on: https://go-review.googlesource.com/c/sys/+/258058
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mksysnum.go

@@ -5,9 +5,8 @@
 // +build ignore
 
 // Generate system call table for DragonFly, NetBSD,
-// FreeBSD, OpenBSD or Darwin from master list
+// FreeBSD or OpenBSD from master list (for example,
-// (for example, /usr/src/sys/kern/syscalls.master or
+// /usr/src/sys/kern/syscalls.master or sys/syscall.h).
-// sys/syscall.h).
 package main
 
 import (
@@ -161,12 +160,6 @@ func main() {
 					text += fmt.Sprintf("	%s = %s;  // %s\n", name, num, proto)
 				}
 			}
-		case "darwin":
-			if t.Match(`^#define\s+SYS_(\w+)\s+([0-9]+)`) {
-				name, num := t.sub[1], t.sub[2]
-				name = strings.ToUpper(name)
-				text += fmt.Sprintf("	SYS_%s = %s;\n", name, num)
-			}
 		default:
 			fmt.Fprintf(os.Stderr, "unrecognized GOOS=%s\n", goos)
 			os.Exit(1)